As a fuel cell system that has as its energy source a fuel cell that generates power from the electrochemical reaction between fuel gas and oxidizing gas, for example, JP Patent Publication No. 2004-165058 discloses a technique of performing so-called power-generation-recovery processing that, while the system is warming-up after activation, if the fuel cell's output voltage is judged to be low, and this drop in output voltage is judged to be caused by residual nitrogen in the fuel cell, increases the amount of fuel gas supplied to the fuel cell.